Microelectronic components, such as semiconductors, generate substantial heat which must be removed to maintain the component""s junction temperature within safe operating limits. Exceeding these limits can change the performance characteristics of the component and/or damage the component. The heat removal process involves heat conduction through an interface material from the microelectronic component to a heat sink. The selection of the interface material and the thermal resistance at the interface between the heat generating component (e.g. silicon ic chip) and the heat sink controls the degree of heat transfer. As the demand for more powerful microelectronics increase so does the need for improved heat removal.
The thermal resistance between the microelectronic component package and the heat sink is dependent not only upon the intrinsic thermal resistance of the interface material but also upon the contact interface thermal resistance formed at the junction between the interface material on each opposite side thereof and the microelectronic component and heat sink respectively. One known way to minimize contact thermal resistance at each interface junction is to apply high pressure to mate the interface material to the microelectronic package and heat sink. However, excessive pressure can create detrimental and undesirable stresses. Accordingly, the application of pressure is generally limited so as not to exceed 100 psi and preferably below about 20 psi.
It is also known to use a thermal grease or paste as the thermal interface material or to use a thin sheet composed of a filled polymer, metallic alloy or other material composition having phase change properties. A material having phase change properties is characterized as having a viscosity responsive to temperature with the material being solid at room temperature and softening to a creamy or liquid consistency as the temperature rises above room temperature. Accordingly, as the microelectronic component heats up the material softens allowing it to flow to fill voids or microscopic irregularities on the contact surface of the microelectronic component and/or heat sink. This allows the opposing surfaces between the microelectronic component and heat sink to physically come closer together as the phase change material melts thereby reducing the thermal resistance between them.
Since the microelectronic package and heat sink do not generally have smooth and planar surfaces a relatively wide and irregular gap may form between the surfaces of the microelectronic component and heat sink. This gap can vary in size from less than 2 mils up to 20 mils or greater. Accordingly, the interface material must be of adequate thickness to fill the gap. The use of thermal grease, paste or phase change materials cannot presently accommodate large variations in gap sizes. In general as the thickness of the interface material increases so does its thermal resistance. It is now a preferred or targeted requirement for a thermal interface material to have a total thermal resistance, inclusive of interfacial contact thermal resistance, in a range not exceeding about 0.03xc2x0 C.-in2/W at an applied clamping pressure of less than 100 psi and preferably less than about 20 psi. Heretofore thermal interface materials did not exist which would satisfy this targeted criteria.
A multi-layer solid structure and method has been discovered in accordance with the present invention for forming a thermal interface between a microelectronic component package and a heat sink possessing low contact interfacial thermal resistance without requiring the application of high clamping pressure. Moreover, the multi-layer structure of the present invention has thermal resistance properties which do not vary widely over a gap size range of between 2-20 mils.
The multi-layer structure of the present invention is solid at room temperature and, comprises a structure having at least two superimposed metallic layers, each of high thermal conductivity with one of the two layers having phase change properties for establishing low thermal resistance at the interface junction between a microelectronic component package and a heat sink and with the thickness of the layer having phase change properties being less than about 2 mils. High thermal conductivity for purposes of the present invention shall mean a thermal conductivity of above at least 10 W/m-K. The preferred class of high thermal conductivity metal carrier layers should be selected from the transition elements of row 4 in the periodic table in addition to magnesium and aluminum from row 3 and alloys thereof.
The preferred multi-layer structure of the present invention comprises at least three layers having an intermediate solid core of a high thermal conductivity metal or metal alloy and a layer on each opposite side thereof composed of a metallic material having phase change properties. A metallic material having phase change properties shall mean for purposes of the present invention a low melting metal or metal alloy composition having a melting temperature between 40xc2x0 C. and 160xc2x0 C. The preferred low melting metal alloys of the present invention should be selected from the group of elements consisting of indium, bismuth, tin, lead, cadmium, gallium, zinc, silver and combinations thereof. An optimum low melting alloy composition of the present invention comprises at least between 19 wt %-70 wt % indium and 30 wt %-50 wt % bismuth with the remainder, if any, selected from the above identified group of elements.
Another embodiment of the multi-layer structure of the present invention comprises a structure with at least one solid metallic layer of high thermal conductivity and a second layer having phase change properties for establishing low thermal resistance at the interface junction between a microelectronic component package and a heat sink, with said second layer superimposed on a surface of said solid metallic layer such that a border of said solid metallic layer is exposed substantially surrounding said second layer. A preferred three layer structure includes an intermediate solid metallic core with two opposing low melting alloy layers on opposite sides with each low melting alloy layer superimposed on a given surface area on each opposite side of said solid metallic core so as to form an exposed border of said solid core extending substantially about said low melting alloy.
A preferred method of the present invention for forming a thermal interface material comprises the steps of: forming a sheet of a high thermal conductivity material of predetermined geometry and thickness, treating at least one of said surfaces to form a treated surface adapted to adhere to a low melting alloy and laminating a layer of a low melting alloy upon said treated surface with the low melting alloy having a thickness of no greater than about 2 mils. The preferred method of treating the surfaces of the high thermal conductivity material to promote adhesion to a low melting alloy layer includes the step of forming dendrites on said high conductivity material which promotes adherence to the low melting alloy during lamination. Another preferred method of the present invention for forming a thermal interface material comprises the steps of: forming a sheet of high thermal conductivity material of predetermined geometry and thickness with the sheet having two opposite surfaces, treating at least one of said opposite surfaces with an organic acid flux adapted to form a treated surface to which a low melting alloy will adhere when coated thereupon, and submersing said sheet into a molten composition of a low melting alloy to form a thin coating of said low melting alloy on said treated surface with said thin coating having a thickness between 0.1 mil and 3 mils.